Terahertz (THz) technologies open great potential in a number of fields, such as spectroscopy, material science, security screening and high-speed wireless communication for example.
The development of waveguides with low dispersion propagation of broadband THz pulses as well as low losses at frequencies above 1 THz is essential to enable undistorted propagation of sub-picosecond pulses and to realize interconnections for future THz communication network, enhanced THz-time spectroscopy (TDS) and sensing technology. Choosing the appropriate material and the suitable geometry for the fabrication of THz waveguides is becoming a main challenge. Furthermore, depending on the geometry, efficient coupling of broadband propagating THz waves to the waveguide can be an issue due to a large mismatch between the mode of the waveguide and the free propagating incident mode.
To date, several THz waveguides have been reported based on dielectric and metallic structures. On the one hand, dielectric based waveguides such as sapphire fibers [1], plastic ribbon waveguides [2] and subwavelength fibers [3, 4] have been developed. On the other hand, metallic waveguides such as single wire waveguides [5, 6], parallel plate waveguides (PPWG) [7] and two-wire waveguides [8-11] can propagate single cycle THz pulses with low dispersion due to their ability to support an almost non-dispersive transverse electromagnetic mode (TEM).
The linearly polarized TEM mode of a two-wire waveguide can be easily excited via a photoconductive (PC) antenna and is characterized by low bending losses, in contrast to single wire waveguides [8]. In addition, a two-wire waveguide provides a tight two-dimensional confinement of the TEM mode and can thus be employed for guiding over longer distances. Recently, a metal-dielectric air-core fiber with two embedded indium wires [12] and a two-wire waveguide structure supported by porous dielectric fibers [13] have been demonstrated experimentally and theoretically, respectively.
In most of the cases, dielectric waveguides are not suitable because of their low dispersion propagation of THz pulses due to the inherent dispersive properties and losses at frequencies above 1 THz. Although hollow core dielectric fibers can boost low-loss and low-dispersion, they are limited in bandwidth due to resonance or bandgap effects [14]. Single wire waveguides carrying radially polarized TEM modes are difficult to excite from commonly available linearly polarized THz sources like PC antennas due to mode mismatch and hence it is necessary to make use of a radially polarized THz radiation source [15]. Furthermore, single-wire waveguides are characterized by high bending losses, which limit their flexibility. Although the low dispersion modes of parallel plate waveguides (PPWG) can be conveniently excited by a commonly available PC antenna, such waveguides cannot be used for long propagation distances. This is due to the one dimensional THz confinement in these waveguides which leads to beam expansion in the unguided dimension, and hence subsequent loss due to diffraction.
Despite the promising results obtained with two wire waveguides, one issue that still needs to be addressed is how to efficiently couple THz pulses into the two-wire guiding structure.
Both the TEM mode supported by a two-wire waveguide and the THz radiation generated by a PC antenna are linearly polarized. The combined system consisting of a PC antenna interconnected with a two-wire waveguide is therefore a very effective solution for the efficient generation, coupling and routing of the THz signal. In earlier work [8, 16], the two-wire waveguide mode was excited by approaching a PC antenna close to the input of the waveguide. In this configuration, it can be assumed that the THz radiation is being coupled into the waveguide from free space, similar to the coupling demonstrated numerically in [9] by considering a single dipole source. However, a large fraction of this free space THz radiation, emitted by the PC antenna, is not coupled into the waveguide and the low, far-field mediated coupling efficiency of the system strongly limits its applicability. The main limiting factor is the difficulty of focusing the THz radiation at the input of the two-wire waveguide as the size of the gap between the wires is close to the diffraction limit.
There is still a need in the art for a method and system for transmitting Terahertz.